If you notice that media thumbnails are not displaying correctly, here is the first thing to check:

Check WordPress Thumbnail Settings

In your WordPress admin panel, go to: Wp-admin – Settings – Media

We use the default WordPress thumbnails generated through the Media settings. If these are missing or blank, the thumbnails won’t display properly.

Your settings must follow the default WordPress thumbnail configuration – exactly as in this screenshot

After settings are changed, existing images thumbnails must be regenerated. The easiest is to use Force Regenerate Thumbnails plugin from WordPress Free Plugins repository